2.8 Java1616: polymorphism and interfaces, part 1

Revised: Thu Mar 31 14:33:41 CDT 2016

This page is included in the following Books:

• ITSE 2321 - Object-Oriented Programming (Java)
• Object-Oriented Programming (OOP) with Java

Table of contents

• Preface
  • Viewing tip
    • Listings
• Preview
• Discussion and sample code
• Summary
• What's next?
• Miscellaneous
• Complete program listings

Preface

This module is one of a series of modules designed to teach you about the essence of Object-Oriented Programming (OOP) using Java.

Viewing tip

I recommend that you open another copy of this document in a separate browser window and use the following links to easily find and view the listings while you are reading about them.

Listings

• Listing 1 . Definition of interfaces named I1 and I2.
• Listing 2 . Definition of the class named A.
• Listing 3 . Definition of the class named B.
• Listing 4 . Definition of the class named C.
• Listing 5 . The driver class named Poly05.
• Listing 6 . Complete program listing.

Preview

Method overloading

I covered method overloading as one form of polymorphism (compile-time polymorphism) in a previous module. I also explained automatic type conversion and the use of the cast operator for type conversion in a previous module.

Method overriding and class inheritance

I also discussed runtime polymorphism implemented through method overriding and class inheritance in previous modules.

Using the Java interface

In this module and the next, I will explain runtime polymorphism as implemented using method overriding and the Java interface.

A very important concept

In my opinion, this is one of the most important concepts in Java OOP, and the one that seems to give students the greatest amount of difficulty. Therefore, I will try to take it slow and easy. As usual, I will illustrate the concept using sample programs.

I will also tie this concept back to the concept of polymorphism using method overriding through inheritance.

A skeleton program

In this module, I will present a simple skeleton program that illustrates many of the important aspects of polymorphic behavior based on the Java interface.

Multiple inheritance and the cardinal rule

I will explain how the implementation of interfaces in Java is similar to multiple inheritance. I will explain the cardinal rule of interface implementation.

A new relationship

I will explain that objects instantiated from classes that implement the same interface have a new relationship that goes beyond the relationship imposed by the standard class hierarchy.

One object, many types

I will explain that due to the combination of the class hierarchy and the fact that a class can implement many different interfaces, a single object in Java can be treated as many different types. However, for any given type, there are restrictions on the methods that can be called on the object.

Many classes, one type

I will explain that because different classes can implement the same interface, objects instantiated from different classes can be treated as a common interface type.